MOCVD-grown p-(Al,In)GaN forms Mg—H complexes that reduce the number of free holes available for conduction and thereby increase the resistivity of the p-(Al,In)GaN layer. The introduction of hydrogen into the p-(Al,In)GaN layer is unavoidable during MOCVD growth because even if the deposition carrier gas does not include H2, the dissociation of NH3 provides sufficient H2 for Mg—H complexes to form.
After the passivated p-(Al,In)GaN layer is formed, Mg—H complexes can be removed by thermally annealing the p-(Al,In)GaN layer in a H2-free environment such as in an N2 and/or O2 environment. The annealing process breaks Mg—H bonds, removes H2 from the p-(Al,In)GaN layer and results in a decreased resistivity. The process of conditioning a passivated p-(Al,In)GaN layer to decrease the resistivity is referred to as activating the p-(Al,In)GaN layer, and the resulting p-(Al,In)GaN layer is referred to as an activated p-(Al,In)GaN layer.
Re-exposing an activated p-(Al,In)GaN layer to a H2 and NH3 environment can cause Mg—H complexes to re-form and thus re-passivate the p-(Al,In)GaN layer.
In general, it is not possible to activate a passivated p-(Al,In)GaN layer after an overlying semiconductor layer such as an n-(Al,In)GaN layer has been deposited on the passivated p-(Al,In)GaN layer. Because H2 cannot diffuse vertically through an overlying n-(Al,In)GaN layer, the buried p-(Al,In)GaN layer cannot be thermally activated when annealed at high temperatures in a H2-free environment.
To activate a buried passivated p-(Al,In)GaN layer, trenches can be etched into the semiconductor structure to expose the edges of the buried p-(Al,In)GaN layer to enhance the ability of H2 to laterally diffuse and escape from the sidewalls of the trench during an annealing step.
Alternatively, activated p-(Al,In)GaN layers can be directly grown using certain semiconductor growth methods in which H2 is not present during the growth process. For example, molecular beam epitaxy (MBE) in which the H2 partial pressure is low, can be used to grow high-quality activated p-(Al,In)GaN layers. However, an activated p-(Al,In)GaN layer can become passivated when exposed to H2 at high temperature, for example, when semiconductor layers are grown over the activated p-(Al,In)GaN layer at higher growth pressure. This can occur, for example, when an activated p-(Al,In)GaN layer is part of a semiconductor device and subsequently deposited semiconductor layers are grown using MOCVD, which employs H2 and/or NH3 as a carrier gas. For these reasons, it has not been possible to grow a semiconductor structure incorporating a buried activated p-(Al,In)GaN layer where the semiconductor layers immediately above the p-(Al,In)GaN layer are grown using metal organic chemical vapor deposition (MOCVD).
Although this can be achieved using RFMBE. and possibly NH3 molecular beam epitaxy (MBE), using these growth methods is undesirable due to the complexities of ultra-high vacuum (UHV) and the difficulty in scaling.
Because semiconductors can be grown in a H2-free environment using RPCVD it should, in principle, be possible to overgrow an activated p-(Al,In)GaN layer using RPCVD without passivating the underlying activated p-(Al,In)GaN layer. However, it has been demonstrated that semiconductor layers grown in a predominantly H2-free environment using RPCVD including p-(Al,In)GaN and n-(Al,In)GaN layers exhibit inferior quality compared to those grown in the presence of H2 and NH3, and consequently viable devices cannot be readily grown using RPCVD without using H2 and NH3. It has also been shown that the levels of H2 and NH3 required during the growth of semiconductor layers by RPCVD to achieve high-quality layers are sufficient to passivate a previously activated p-(Al,In)GaN layer.
Therefore, although semiconductor layers overlying an activated p-(Al,In)GaN layer can be grown in a H2-free environment such as by using RPCVD and can be expected to retain the activation state of the p-(Al,In)GaN layer, the reduced quality of the overlying semiconductor layers due to the growth conditions will negatively affect device performance. In contrast, the quality of the overlying layers can be improved through the use of sufficient amounts H2 and NH3 during the RPCVD growth. However the levels of H2 and NH3 that are required to achieve the highest quality would then be expected to passivate any underlying p-(Al,In)GaN layers. It would therefore be expected that for any device comprising a buried p-(Al,In)GaN layer with overlying layers grown using an optimized RPCVD process, the buried p-(Al,In)GaN will not be fully activated.
It is desirable to have a semiconductor growth process in which semiconductor layers can be grown overlying an activated p-(Al,In)GaN layer in a high H2 environment such as using MOCVD without passivating the underlying p-(Al,In)GaN layer and without having to undertake post-fabrication steps to re-activate the buried passivated p-(Al,In)GaN layer.